The present invention relates to an ink ejection apparatus for an ink jet printer comprising means for removing entrained air from the ink.
An ink jet printer of the so-called demand type comprises an ink reservoir which communicates with the atmosphere, an ink ejection head and conduit connecting the reservoir to the head. The ink ejection head comprises a piezoelectric vibrator. To print a dot of ink, an electrical signal is applied to the vibrator which causes the same to vibrate. Expansion of the vibrator causes a pressure increase in the head which ejects ink onto a sheet of printing paper. Compression of the vibrator causes a pressure decrease which sucks ink into the head from the conduit.
A problem arises if air, especially in the form of bubbles, is entrained in the ink. The ink, due to the air, becomes compressible and is not readily ejected from the head. This is because the air compresses and absorbs the pressure applied to the ink by the vibrator.
Air is able to enter the system from the tank, since the tank is generally open to the atmosphere, especially during filling. This problem is compounded by the fact that the ink in the conduit is not pressurized. Ink can also be sucked into the ink ejection head during compression of the vibrator, generated by organic materials in the ink and temperature variations.
Various proposals have made in the prior art for removing entrained air from ink in a demand type ink jet printing apparatus. One is described in Japanese Utility Model Publication No. 52-38215 which comprises a disposable ink cartridge. While this system prevents air from entering the cartridge, it cannot prevent air from entering the conduit and ink ejection head. The disposable cartridges constitute an unnecessary expense.
Another means for removing entrained air is disclosed in Japanese patent application Nos. 53-42033 and 53-45239. Such a system comprises an air trap or filter which is designed to remove bubbles from ink flowing therethrough. However, disadvantages are present in that the flow rate through the filter decreases as the amount of trapped bubbles increases, thereby reducing the amount of ink ejection. The problem may only be overcome using means for sensing the flow rate through the filter and producing a signal when the flow rate drops below a predetermined value. In response to the signal the filter must be replaced or purged. Such a manual operation is quite undesirable.
Another prior art system is disclosed in Japanese patent application No. 52-49032 in which a piston or plunger is depressed to pressurize the ink and force air and bubbles into a collection chamber formed at the top of the ink ejection head. This system suffers from the disadvantages that a mechanism must be provided to periodically operate the piston and that the system is not easily adapted to an ink ejection apparatus comprising a plurality of ink ejection heads supplied from a single reservoir.